Opto-electronic devices including light emitting diodes (LEDs), laser diodes and the like, are an ever-increasing part of everyday life. Indeed, when people go through the course of a single day of their lives, they will encounter perhaps hundreds or even thousands of LEDs and laser diodes.
Commonplace examples of the use of LEDs include alarm clock displays, simple indicators on consumer electronic devices, large-scale TV-like displays, and even traffic signals at street intersections. Examples of the use of laser diodes include supermarket barcode scanners, laser pointers, construction alignment devices, and police traffic radar.
Most opto-electronic devices can be easily damaged if their nominal voltage or current parameters are exceeded. In fact, products that contain opto-electronic devices often seem to mysteriously fail, with no apparent provocation. A close examination into the failure modes of these devices has revealed power surges during power-up/power-down sequences and electrostatic discharge (ESD) events as two major causes of opto-electronic device failure.
When power is being turned on or off on a product, internal circuits can be operating outside their intended internal power supply range for a brief period of time. As one example, rail-to-rail operational amplifiers used in laser diode drivers may be specified to operate with power supplies ranging from 2.7 to 5.5 volts. But the manufacturer makes no guarantee or representation regarding what the operational amplifier will do between a supply voltage of 0 and 2.7 volts. Because of this, current or voltage regulation circuits may go out of balance during power-up and power-down, and often apply an over-current or over-voltage condition to the opto-electronic device that is integrated within the product. These over-current or over-voltage conditions may stress the opto-electronic device, such that each power-up or power-down cycle accumulates in the form of device fatigue. Eventually, the opto-electronic device may fail from the fatigue, leading to what the user experiences as a mysterious failure.
Electrostatic discharge (ESD) may also cause mysterious device failures, and there are many ways in which ESD might come in contact with a product. One of the most common ways that ESD is generated occurs when a human walks across a carpeted floor, and then touches something. A discharge can occur to anything being touched, ranging from things that are not sensitive to ESD, such as doorknobs, to things that are very sensitive to ESD, such as electronic products.
Studies have shown that, when an ESD discharge occurs, the discharge voltage can range anywhere between 4000 and up to as much as 32,000 volts DC, depending on environmental conditions, clothing being worn, type of flooring surface, and other factors. When an opto-electronic device that is designed to operate with a terminal voltage of 2.2 volts experiences an ESD discharge of thousands of volts, the result can be destructive.
Laser diodes typically fail as the result of two distinct damage mechanisms. One of the damage mechanisms is optically related, and occurs when the laser diode is producing light (referred to as “lasing”), and the optical energy density exceeds the laser diode's integral mirrors' reflective capacity. When this occurs, the mirrored surface permanently loses its reflectivity, and the laser diode no longer functions properly. This could be thought of as the laser light becoming so intense that it vaporizes the mirror surface. The second damage mechanism is related to failure of a laser diode's PN junction itself. A severe over-current or over-voltage power surge can cause localized heating and other harmful phenomena, which, under extreme conditions, can actually fracture the laser diode die. Obviously, when this happens, the laser diode will no longer function. Both of these damage mechanisms can be provoked by an over-voltage or over-current condition.
Low-power laser diodes, that is, laser diodes whose optical output power is below around 200 mW, are particularly sensitive to ESD. This is because they are designed to be inherently fast devices. Indeed, low-power laser diodes are often directly modulated and used for fiber-optic communication with data rates in the gigahertz range. Thus the PN junction and optical elements of a laser diode can react very quickly to changes in voltage or current. Therefore, in order to be effective, an ESD protection device and method should preferably be implemented as a proactive measure, by preventing the over-voltage or over-current condition from happening in the first place, not by reacting to it once it has occurred.
FIG. 1 illustrates the current vs. voltage profile of a typical low-power laser diode. It can be seen that the profile is similar to other types of diodes and semiconductor devices. Starting from zero volts, applying incremental positive increases in voltage (i.e., those voltages that would tend to forward bias the laser diode), very little current flows until around 1.8 volts is reached. Further incremental positive increases from around 1.8 volts causes current flow to increase at a roughly exponential rate. However, the laser diode does not emit laser light until the current exceeds a “lasing threshold,” which, for the laser diode referred to here, occurs at around 30 milliamps and at around 2.2 volts.
With further incremental positive increases in voltage, current flow continues to increase, while the optical power emitted by the laser diode increases at a rate that is roughly proportional to current. Once the maximum design current for a particular laser diode is reached (which is around 35 milliamps and 2.4 volts for this laser diode), further increases in current will likely result in failure, caused by one or both of the damage mechanisms described above. Thus it is important to completely prevent voltage, and thus current, from increasing beyond the absolute maximum rating for a particular diode. In most cases, a low-power laser diode will be destroyed if the absolute maximum ratings are exceeded, even for a brief period of time.
Herein the term “positive-ESD” is used to mean ESD whose voltage polarity would tend to forward-bias a laser diode, and “negative-ESD,” to mean ESD whose voltage polarity would tend to reverse-bias a laser diode.
Note that FIG. 1 illustrates only the current vs. voltage profile for positive voltages, that is, voltages that would forward-bias the laser diode. Laser diode manufacturers recommend that negative voltages, that is, voltages that would tend to reverse-bias the laser diode, be avoided. The data sheet for an exemplary laser diode lists an absolute maximum reverse voltage of 2 volts.
In order to protect this laser diode from being damaged by ESD, the protection means should limit positive voltages to around 2.4 volts and negative voltages to around 2.0 volts or less. These voltages are used as a reference throughout the rest of this discussion.
In order to evaluate the effectiveness of an ESD protection scheme, it is useful to employ an electrical circuit model that helps to illustrate and understand the voltage and current levels that are experienced during an ESD event. There are several so-called “human body models” for the evaluation of ESD, and a useful one is shown in FIG. 2, wherein a 150-picofarad capacitor charged to a predetermined voltage is shown, in series with a 330-ohm resistor, which is then connected to the device under test (DUT). This may also be the human body model used by the IEC 61000-4-2 standard. As mentioned above, the voltage level of an ESD event ranges from around 4000 to around 32,000 volts DC. However, for the purpose of modeling ESD events, it is common to use a representative voltage of 15,000 volts.
In looking at the human body model, with the capacitor charged up to 15,000 volts, discharging through a laser diode using the 330-ohm series resistance of the human body model, it can be seen that the laser diode will experience a current in excess of 45 amps during the discharge. And a simple R/C analysis shows that this discharge happens over a very short period of time, no greater than tens of nanoseconds. Real-world ESD events have been observed in the one- to two-nanosecond range. Thus, in order to be effective, an ESD protection means must react in the nanosecond range, and also have an effective frequency bandwidth that ranges from around 20 MHz to 1 GHz.
Using the human body model shown in FIG. 2 as a guide, it can be seen that, if the ESD protection scheme is implemented as a passive means, and placed in parallel with the laser diode, it must have an effective impedance less than 44 milliohms in order to protect a laser diode whose absolute maximum reverse-bias voltage is 2.0 volts and whose forward-bias limitations are similar. The mathematics are (15,000 volts/(330 ohms+44 milliohms))*44 milliohms=1.99 volts.
Within the current state of the art, there are several ESD protection means employed for the purpose of protecting laser diodes, and in order to locate the protection means close to the laser diode, these protection means are often embodied within a “head” that may be located remotely from the laser diode driver circuitry.
FIG. 3 illustrates one of the ESD protection means known to be employed for protecting laser diodes. In this scheme, a resistor is connected directly across the laser diode terminals, within the head, and with the resistance being typically around 100 ohms.
Using the human body model in FIG. 1 as a guide, it is easy to see that this protection means will not be effective. As discussed above, for a 15,000 volt ESD event, the resistance would need to be less than 44 milliohms, in order to prevent the laser diode voltage from exceeding the exceeding the maximum reverse bias voltage of 2.0 volts and exceeding a similar forward-bias voltage. If a 100-ohm resistor is used, it would allow (15,000 volts/(330 ohms+100 ohms))/100 ohms=3488 volts to surge into the laser diode. Since this is far in excess of the typical 2.2-volt lasing threshold or 2.0-volt maximum reverse bias voltage, this would almost surely destroy the laser diode. Although it might seem that the 100-ohm resistor could simply be replaced with a 44-milliohm resistor, this is not practical, because it would mean that, during operation, far more power would be expended in the operation of the protection resistor than the laser diode itself.
FIG. 4 illustrates a similar scheme that is currently in use, but one in which a capacitor is used instead of a resistor. In this application, typical capacitor values range from several hundred nanofarad to several microfarad. At first glance, a 1-microfarad capacitor would appear to be sufficient to limit a 15,000-volt ESD event from exceeding the maximum reverse bias voltage of 2.0 volts and similar forward-bias limits. However, in real life, there are no capacitors known to exist that have purely capacitive characteristics.
All known real-world electrical components have parasitic properties. Small capacitors can be modeled by the nominal capacitance in series with a parasitic resistance in series with a parasitic inductance. As discussed, an ESD event occurs within the range of a few nanoseconds up to a few tens of nanoseconds; thus, the frequency-domain equivalent of this is around 20 MHz up to 1 GHz. Therefore, the impedance of the capacitor would need to be less than 44 milliohms between around 20 MHz and 1 GHz in order to be effective at protecting the laser diode. Common 1-microfarad electrolytic capacitors have an equivalent series resistance of 1 ohm, and an equivalent series inductance of around 15 nanohenry. This combination clearly gives an impedance greater than 44 milliohms. And although the best 1-microfarad tantalum capacitors have an equivalent series resistance that can approach 50 milliohms, their equivalent series inductance is usually at least 1 nanohenry, which gives an impedance over 6 ohms at 1 GHz. It is not known whether a capacitor actually exists whose impedance is 44 milliohms within the frequency range of interest. However, even if a perfect capacitor were used that would be effective at protecting the laser diode against ESD, such a capacitance makes direct modulation of a laser diode increasingly difficult, especially at high modulation frequencies. Thus, there are clear drawbacks to this simple capacitive approach.
FIG. 5 illustrates another ESD protection means commonly employed to protect laser diodes. In this scheme, a Schottky diode is placed in parallel with the laser diode. However, there are several problems with this technique. First of all, most Schottky diodes were not designed to handle nanosecond pulses of up to 50 amps. Within the present inventor's own testing using the human body model, many Schottky diodes were themselves destroyed by an ESD event. Therefore, if the device that is put in place as the ESD protection means is destroyed by the ESD event, this is deemed to be an ineffective ESD protection means. Second, and more important, such a configuration would only protect the laser diode from negative-ESD events (i.e., those events that would tend to reverse-bias the laser diode). Positive-ESD events are allowed to pass through to the laser diode without being attenuated by a Schottky diode. Thus, at best, a Schottky diode is an incomplete ESD protection means.
FIG. 6 illustrates a similar approach, but one in which a Zener diode is used in place of a Schottky diode. There are several problems with this technique. One problem is that Zener diodes are notoriously slow devices, and are not able to react to nanosecond-level pulses that can be seen during ESD events. The second, and more serious, problem is that it is believed to be impossible to choose a Zener voltage that is close enough to the forward-bias lasing threshold of the laser diode, and whose Zener voltage would track the laser diode's lasing threshold voltage throughout temperature ranges likely to be experienced by a laser diode.
FIG. 7 illustrates another approach commonly employed to protect a laser diode from ESD. In this approach a “switch” is used, and placed across the terminals of the laser diode. This “switch” is often implemented as a relay, but implementations are known in which this “switch” is implemented as a depletion-mode metal oxide semiconductor field effect transistor (MOSFET). The idea is that this “switch” would be “normally closed”; that is, while no power is applied to the laser diode or laser diode driver circuitry, the “switch” shorts the terminals of the laser diode such that ESD is conducted by the “switch” rather than the laser diode.
Having an ESD protection means that is effective when no power is applied to the system is indeed desirable, because very often, ESD events happen while the system power is turned off. However, this “switch” approach has several important drawbacks, which will be discussed separately for the case of the relay and for the depletion-mode MOSFET.
As discussed, the impedance of any passive technique would need to be less than 44 milliohms in order to be effective in preventing a 15,000-volt ESD event from exceeding the typical 2-volt maximum reverse-bias voltage and similar forward-bias limitations of a typical low-power laser diode. If this “switch” is implemented as a relay, throughout the life of the relay, the contact resistance, along with any printed circuit board (PCB) traces and other interconnections that lead from the relay to the laser diode, need to be collectively less than 44 milliohms.
As relays open and close over and over during their lifetime, their contacts wear, and it is possible that as the relay ages, the contact resistance plus interconnect resistance could exceed 44 milliohms. Moreover, during an ESD event in which the relay is closed, up to 50 amps or more could be conducted by the relay contacts. Repeated ESD events could lead to fretting corrosion of the relay contacts, and eventual failure of the relay. Moreover, during an ESD event, a magnetic field is set up around the relatively long leads within the relay geometry, along with the contacts themselves. This magnetic field could couple to nearby PCB traces, and to the relay coil, effectively coupling the ESD to other parts of the circuit that could also be sensitive to ESD. Thus, even if the laser diode itself were protected, ESD could prove destructive to the laser diode drive circuitry.
When a depletion-mode MOSFET is used as the “switch” and when the power is off, the gate and source terminals are at the same (zero) voltage potential. This turns a depletion-mode MOSFET “on,” thus helping to conduct ESD across the terminals of the MOSFET instead of the laser diode. Unfortunately, the typical on-resistance of a depletion-mode MOSFET is in the range of several ohms. An exemplary device has a RDS(on) of 6 ohms. As discussed, this resistance would need to be less than 44 milliohms in order to protect a typical laser diode from a 15,000-volt ESD event. Thus, a depletion-mode MOSFET would not be an effective ESD protection means for 15,000-volt ESD events.
Whether the “switch” is implemented as a relay, MOSFET, or some other device, there is another drawback to this approach. The “switch” approach is generally applicable to systems whose power is turned off. Once the system power is turned on, the switch is opened and the laser diode is allowed to become operational. If an ESD event happens while the laser diode is operational and lasing, the “switch” will have no effect, and will not protect the laser diode from ESD.
FIG. 8 illustrates yet another approach commonly employed to protect a laser diode from ESD. In this approach a multi-layer varistor is placed in parallel with the laser diode. A multi-layer varistor is a device whose resistance changes, decreasing nonlinearly, with increases in voltage that appear across the terminals. Although multi-layer varistors have response times in the nanosecond range, their breakdown voltage (the voltage at which the varistor transitions from high resistance to low resistance) is typically well in excess of the 2.2-volt lasing threshold, or 2.0-volt maximum reverse-bias voltage of a typical low-power laser diode. A multi-layer varistors whose breakdown voltage is below 3.6 volts is not known to the present inventor. Therefore, this approach is not believed by the present inventor to be completely effective in preventing a low-power, fast-response laser diode from being damaged by 15,000-volt ESD.